72 



ANIMAL AGGREGATIONS 



minutes after the background was moistened. The animals not 

 shown in this photograph have crawled up the sides of the dish. 



A somewhat similar effect of drought in nature is reported for the 

 California quail (Evermann, 1901). In an unusually dry season these 



quail do not breed but remain in 

 flocks during the entire summer. 

 The opposite type of moisture 

 control is also observable. Too 

 much moisture may produce 

 well-defined aggregations. Thus 

 Solenopsis geminata (von Ihering, 

 1894), a species of ant which 

 often nests in lowlands, will, if 

 the nest is flooded, aggregate in 

 a ball of some 15-20 cm. in di- 

 ameter, with the larvae and 

 pupae inside. By constant rota- 

 tion they avoid too long sub- 

 mergence, and at length may 

 come against some solid object 

 and so escape from the water. 

 Wheeler (1913a) cites this case 

 and mentions similar instances 

 in this and other species of ants. 

 The formation of the dancing 

 bunches of midges already men- 

 tioned, which one frequently 

 sees aggregated in the space of 

 a half -bushel basket, appear to 

 be in part conditioned by the at- 

 mospheric humidity, although 

 the absence of wind is another 

 obvious prerequisite. 

 In both these cases the environmental conditions are uniform; and 

 the animals, in grouping together, react to each other only. There 

 are also the place aggregations controlled by moisture, when animals 



Fig. 3. — (i) Land isopods in darkened 

 room on dry background of filter paper. (2) 

 Same animals and conditions as in (i) except 

 that the filter paper has been moistened. 



